Seismology: Fundementals 2

Introduction to Seismology
Introduction to Seismology-KFUPM

Strong Motion Seismology
Ali Oncel
oncel@kfupm.edu.sa
Department of Earth Sciences
KFUPM

Previous Lecture

Richter's Local Magnitude
Earthquake Magnitudes
Magnitude Saturation
What causes saturation?
Are Mb and Ms still useful?
What is the better estimate of M?
Moment magnitude
Strength of Earthquake
Magnitude-Energy Correlation

How to compute the energy
released by an earthquake?
E = Mo/(2 x 104)

log E = 1.5 Mw + 11.8

Where Es is seismic energy
This figure was produced in cooperation with the US
Geological Survey, and the University of Memphis private in ergs. Then,
foundations

log Mo - log(2 x 104) = 1.5 Mw + 11.8
Mw = (log Mo - 16.1) / 1.5

1 erg= 10-7 Joule
From: Kanamori, H., 1977, The Energy Release in Great Earthquakes, Journal of
Geophysical Research, v82, p. 2981- 2987.

1


Example: Energy for Mw=4.0?
The total seismic energy radiated from the source,
E(4), would be:
E(4) = 10**(1.5*4 + 11.8)
= 10**17.8 ergs
= 6.3 x 1011 Joules 1 erg= 10-7 Joule
The moment, Mo(4), would be:
Mo(4) = E x (2 x 104)
= 1.26 x 1015 Joules
It has been found that a 1 kton explosion will generate
seismic waves approximately equivalent to a magnitude 4
earthquake.
The energy released by TNT (trinitrotoluene) and the TNT equivalent of the Hiroshima
nuclear bomb (McGraw-Hill Encyclopedia of Science and Technology, 1992):

Energy per ton of TNT = 4.18 x 109 Joules

Earthquake
Damage

A rescue worker stands by a crack in Parks Collapse of Fourth Avenue near C Street, Anchorage, due to
Highway caused by a 7.9 earthquake that rocked earthquake caused landslide. Before the earthquake, the
a sparsely populated area of Alaska. By Jimmy
sidewalk at left, which is in the graben, was at street level on
Tohill, AP
the right. The graben subsides 11 feet in response to 14 feet o
horizontal movement. Anchorage district, Cook Inlet region,
Alaska. 1964

A crack system destroys driveway
adjacent to summit road 0.8 km (1/2 mi)
southeast of Highway 17. [J.K. Nakata,
U.S. Geological Survey] Aerial view of collapsed sections of
the Cypress viaduct of Interstate
Highway 880. [H.G. Wilshire, U.S. Close-up view of tsunami damage along the waterfront at
Geological Survey] Kodiak.

A Given Earthquake has:
Only one magnitude. A certain amount of energy was
released.
Parks and Plates
Different intensity. Depends on your location. ©2005 Robert J. Lillie

Intensity is a qualitative (non-numerical) rating of the
ground shaking at a given place. It is inferred from the effects
of the earthquake on the land and any buildings in the area.
First-hand accounts from people are also used. Some scales of
intensity are the Modified Mercalli, JMA, and MSK Intensity
Scales.
Quantitative (numerical) measures of ground motion from
accelerograms are descriptions of shaking “severity” and should
not use the word “intensity”.

2


HARRY O. WOOD and FRANK NEUMANN, Modified Mercalli intensity scale of 1931, pp.277-283, BSSA

Felt Report Form
F
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From: http://earthquakescanada.nrcan.gc.ca/dyfi/unknown_form_e.php
n

M=5.6, July 8, 1986, Palm Spring Earthquake

From: http://www.data.scec.org/Module/Pics/s3a1answ.gif

3


Earthquake
Fa

Intensity Generally
ultl

Increases with
ine

Distance from
Epicenter

Parks and Plates
©2005 Robert J. Lillie

Earthquake
Intensity Generally
Increases with
Distance from
Epicenter

Parks and Plates

INTENSITY
Factors that determine how intense the shaking
was at a given location: New Madrid Earthquakes (1811-12)
Magnitude of the earthquake.
Distance from the earthquake
focus.
Local soil conditions.

Earthquake Intensity Database
Search 1638-1985

U.S. Geological Survey map
From: http://www.ngdc.noaa.gov/seg/hazard/int_srch.shtml

4


Animated Example from Hayward Fault
This sequence of intensity
maps shows how the
intensity grows as a fault
rupture increases in length.

This sequence, shown for
the northern Hayward fault,
starts from a hypothetical
magnitude 5 and grows to a
magnitude 7.1 earthquake.

The dynamic rupture process in this hypothetical earthquake
would occur in about 8 seconds.

Damage Scenario

A damage scenario is a representation of the possible
damage caused by an earthquake to the built
environment in an area. It is in terms of parameters useful
for economical and engineering assessment or
postearthquake emergency management.

A probabilistic earthquake scenario is a representation
of earthquake effects. The earthquake has a specified
probability of exceedance during a prescribed period in an
area.
Adapted from the International Handbook of Earthquake
and Engineering Seismology, Aki and Lee[1]
From the International Handbook of Earthquake and Engineering
Seismology, Aki and Lee[1]

Used at Los Angles Co. Emergency Operations Center, By LA
County Office of Emergency Management for Training
Scenario, November 9, 2000.

LA County Emergency
Operations Center

5


Earthquake Damage Scenario
Shake Map: Example for Verdugo Fault

EXPLANATION of the online
"Intensity Maps" activity

Source : http://www.data.scec.org/Module/s3act01.html

Assignment: Measuring Earthquakes
Due to April 15, Group Based Work is suggested.

Exercises
I. Finding Isoseisms
II. Locating an Epicenter
by Creating an Intensity
Map

http://www.data.scec.org/Module/s3act01.html

6


Previous Lecture

Magnitude
How to compute the energy released due to an
earthquake
Earthquake Damage
Magnitude-Intensity
Mercalli Scale of arthquake Intensity
Data Acquisition for Earthquake
Example: Palm Spring Earthquake, M5.6, July 8, 1986
Relationship between Intensity and Distance
Animated Example of Intensity: Hayward Fault
Damage Scenario
Earthquake Planning Scenario: Example for Verdugo Fault

Project and Presentation

Faults
A fault is a fracture
across which there
has been sliding
parallel to the
Parks and Plates
fracture surface.

When people stand across the fault, their heads are next to the
hanging wall, their feet next to the foot wall.
a) Normal fault: A normal fault is a dip-slip fault where the rock
above the fault moves down compared to the rock below the fault.
b) Reverse fault: A reverse fault is a type of dip-slip fault. The
hanging-wall fault block of a reverse fault moves upward relative
to the footwall fault block. (A thrust fault is a low-angle reverse
fault.)
c) Strike-slip fault: A strike-slip fault is a fault where two plates slide
horizontally past each other. It is called a strike-slip fault because
the “slip” of the fault is parallel to the “strike” of the fault.

7


Faults

Strike-slip faulting
Strike-
animation

Strike-Slip Fault

Source: Photo taken from Martin Miller's website, University of Oregon. millerm@darkwing.uoregon.edu

8


Normal Faulting
animation

Thrust
Faulting
animation

Source: University of Victoria.

9


Focal Mechanisms

There are special graphics,
called focal mechanisms or
“beach balls” that we use
as shorthand to describe
the style of faulting.

Seismographs
A seismograph is an instrument which writes a permanent
continuous record of earth motion.[1] Measuring the three
orthogonal components of ground motion at a seismic
station.

Simplified motions of seismic waves

Seismic waves are waves of energy that travel through
Earth and along its surface. The waves are produced by
earthquakes, explosions, or some other disturbance.
Seismic waves are studied to locate and understand
earthquakes and to determine the structure of Earth's
interior.

10


a)Body and surface wave
paths from an
earthquake located SSE
of a station.
b) Seismograms from each of the three
seismometers, responding to arrivals of the body
(P= compressional, S= shear) and surface
(L=Love, R= Rayleigh) waves

Initial Ground Motions and Z-component
seismograms for P-waves

Pushes the ground up

Initial arrival as a compression pushes the ground
up; Z-component shows an upward first motion.

Initial Ground Motions and Z-component
seismograms for P-waves

Pulls the ground down

Initial arrival as a dilatation pulls the ground
down; Z-component shows an downward first
motion

11

Compression

Dilatation

Initial P-wave Radiation Pattern:
from an Earthquake occurring on a reverse fault

a)Waves radiate outward in quadrants of
compression and dilatation
b)The Z-component seismograms for the three
stations highlighted in (a).

a) Map view of radiation pattern for right-lateral, strike-
slip fault occurring along the San Andreas transform
plate boundary
b) First-motion information for arrival at stations indicated
in (a), plotted as a focal mechanism solution.

12


c) Auxiliary fault interpretation of the first-motion in (a),
showing that the same radiation pattern could have
resulted from a left-lateral fault.
d) Focal mechanism solution for (c) is exactly the
same as that resulting from right-lateral fault in (a).

Case Work: Fault interpretation
May 1, 2003
May 1, 2003- Bingol Earthquake (Mw=6.4)
Mw=6.4

What is the strike of fault?

TUBITAK Earthquake Stations

13


Earthquake Aftershocks

Earthquake Aftershocks

Depth Distribution

Observed surfaced deformation due to Bingol Earthquake

14


Homework: VIRTUAL SEISMOLOGY
You have learned how to pinpoint the location of an
earthquake by measuring the speed of seismic
waves radiating away from the focus of the
earthquake. Now, we can determine an earthquake's
magnitude by measuring the strength of ground
shaking as you did for global earthquakes. Learn
how to do both these things by visiting the virtual
earthquake web page.
http://www.sciencecourseware.org/VirtualEarthquak
e/VQuakeExecute.html
and completing the exercise. It should take you
about 30 minutes. Turn in your certificate of
completion at the beginning of class on Monday,
26 March.

15

Seismology: Fundementals 2

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